1. Field of the Invention
This invention relates generally to bipolar circuits that amplify differential input signals and particularly to such circuits suitable for operational amplifiers utilizing relatively low power supply voltages.
2. Description of the Prior Art
The operational amplifier is an important electronic building block. In the form of an integrated circuit, an operational amplifier usually consists of a differential input stage, a biasing circuit, and an output stage. There are a number of difficulties with conventional bipolar operational amplifiers. Their construction is often relatively complex. They occupy large areas on semiconductor chips and typically require high power supply voltages.
A critical part of an operational amplifier is the differential amplifier that receives the differential input signal to the input stage. One approach in designing the differential amplifier is to utilize a combination of NPN and PNP transistors for amplifying the input signal. For example, see J. Solomon, U.S. Pat. No. 3,538,449, where the use of complementary bipolar transistors allows the differential amplifier to operate at a reduced input current level.
Another design approach is to apply the differential input signal to a pair of multi-collector transistors connected to a current mirror in such a way that the amplified signal current does not pass through the mirror. As described by J. Solomon in "The Monolithic Op Amp: A Tutorial Study," IEEE J. Solid-State Circs., Vol. SC-9, December 1974, pp. 314-332, this approach improves the frequency response. However, current from grounded collectors employed to reduce the transconductance and to provide an acceptable common-mode rejection ratio of the input signal is wasted in this Solomon differential amplifier.
Some bipolar operational amplifiers have been designed so as to be operable at relatively low power supply voltages. R. Widlar discloses an operational amplifier of this type in "Low Voltage Techniques," IEEE J. Solid-State Circs., Vol. SC-13, No. 6, December 1978, pp. 838-846. However, these operational amplifiers are disadvantageous in that their circuitry is generally even more complicated. In addition their bandwidths are limited, and their output signals have undesirable distortions. Because of their size and complexity, they have generally not been satisfactory for use as sub-circuit blocks in large scale integration.